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Re: [PATCH] PR target/65248: [5 Regression] Copy relocation in PIE against protected symbol


On Mon, Mar 2, 2015 at 5:39 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Mon, Mar 2, 2015 at 4:05 AM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Mon, Mar 2, 2015 at 11:09 AM, Alan Modra <amodra@gmail.com> wrote:
>>> On Mon, Mar 02, 2015 at 09:40:01AM +0100, Richard Biener wrote:
>>>> On Sat, Feb 28, 2015 at 5:42 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> > Ue copy relocation in PIE improves performance.  But copy relocation
>>>> > can't be used to access protected symbols defined in shared libaries
>>>> > and linker in binutils 2.26 enforces doesn't allow it.  GCC doesn't
>>>> > know if an external definition is protected or not.  This option adds
>>>> > -mcopyreloc-in-pie to give user an option to turn it off to avoid problem
>>>> > at link-time.  OK for trunk?
>>>>
>>>> I wonder if the linker can fix this up?  That is, turn the relocation into
>>>> a valid one?
>>>
>>> No it can't (*), nor can the dynamic linker.  Copy relocs aren't
>>> really the issue.  They are just a means of initializing a linker
>>> generated variable to be used in place of a variable in a shared
>>> library.  The issue is the linker generated .dynbss variable itself.
>>>
>>> Consider an ELF executable linked against a shared library, with the
>>> executable referencing (but not defining) a variable defined in the
>>> shared library.  You'd expect that the executable and shared library
>>> would both use the same location for the variable.  Indeed, that is
>>> true.  Both executable and shared library use the shared library's
>>> variable.  Except there is a wrinkle.  If the executable is non-PIC,
>
>             ^^^^^^^^^^^^
> This is the key here.  This optimization makes PIE behave like normal
> executable. Is it good or bad?
>
>>> code in the executable will require dynamic text relocations as the
>>> variable's address isn't known until run time.  To avoid that, some
>>> clever person thought: "Why not have the linker define the variable in
>>> the executable?  ELF run time linking semantics mean the shared
>>> library will now use the linker defined copy, so we'll still just be
>>> using one copy of the variable".  Any everyone was happy.  At least
>>> until ELF visibility was invented.
>>>
>>> When ELF visibility comes into play, a variable defined in a shared
>>> library with non-default visibility is *not* overridden by another
>>> definition in the executable, be it an actual definition or a linker
>>> generated one.  There is no problem of course if there is an actual
>>> definition in the executable.  In that case the programmer would
>>> expect to see two different variables used.  However, if the shared
>>> library contains a protected visibility variable, and the linker
>>> introduces a copy, then it has changed the meaning of the program.  At
>>> the source level we only had one definition of the variable, but at
>>> run time we'd end up using two different locations.
>>>
>>> *) Except by avoiding .dynbss copies and hence requiring dynamic text
>>> relocations.
>>
>> Ah, I see (protected visibility has haunted us in the past...).
>>
>> So I think we need to turn the new option off by default.
>>

By turning this optimization on by default, we make PIE behave the same
as normal executable and provide a way to change PIE behavior.  This
is more consistent to users.

-- 
H.J.


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